This invention relates to acousto-optic character generators and is more particularly concerned with improvements in acousto-optic character generators of the type disclosed in co-pending U.S. patent application Ser. No. 550,590, filed Nov. 10, 1983 and commonly assigned herewith (incorporated herein by reference) now U.S. Pat. No. 4,540,245.
Broadly speaking, in character generators of the foregoing type a bulk wave acoustic energy representation of a character (or characters) to be reproduced is coupled into a body of acousto-optic material which is then illuminated by a discrete pulse of light, such as a pulsed laser beam. The acoustic wave fronts propagating through the body cause the incident beam to be diffracted, providing a plurality of output beams corresponding to different portions of the character. The output beams are directed to appropriate locations in a character reproduction plane to form an image of the character.
In greater detail, the representation of the character takes the form of one or more sequences of traveling acoustic energy packets which are introduced into respective acoustic energy columns in the acousto-optic body by selectively energizing members of a set of acoustic transducer electrodes mounted on the body. Each sequence of packets represents a respective column of the character, with the individual packets in a given sequence having different predetermined frequencies and corresponding to different portions of the character column.
When one or more character representations have been coupled into the acousto-optic body in the manner just described, a laser beam is pulsed into the body at the so-called Bragg angle. The various acoustic energy packets within the body interact with the incident beam to yield corresponding columns of pulsed diffracted light beams emerging form the body. The direction of each output beam is fixed by the frequency of its corresponding acoustic energy packet in accordance with generally understood acousto-optic principles. The output beams are then directed to a character reproduction plane through a converging lens having a focal plane coincident with the reproduction plane. The converging lens, which may be a spherical convex lens, acts to focus all light rays which enter it in the same direction (i.e., parallel light rays) to a single spot in the reproduction plane, regardless of the rays' points of entry into the lens. Therefore, even though the respective sources of the diffracted beams (i.e., the acoustic energy packets) move through the acousto-optic body while exposed to the laser pulse, the lens directs the beams toward corresponding fixed points (focal points) in the reproduction plane because the directions of the beams are fixed by the frequencies of the respective acoustic energy packets. In order to prevent the converging lens from focussing diffracted beams from adjacent acoustic energy columns to common points (as would occur in the case of adjacent beams originating from packets of the same frequency) and in order to form distinct columns of fixed spots at the reproduction plane, a diverging lens is interposed between the converging lens and the reproduction plane to compensate the tendency of the converging lens to focus transversely to the acoustic energy columns. The resulting columns of spots in the reproduction plane form an image of the character.